Processes for the synthesis of 3-mercaptopropionic acid esters are already known in the patent literature.
Thus, J56 147,763 describes such a process by addition of H.sub.2 S to acrylic acid esters, using anion-exchange resins as catalysts, these resins having tertiary amines or quaternary ammonium hydroxides as functional groups.
U.S. Pat. No. 5,008,432 relates to the addition of H.sub.2 S to unsaturated compounds whose olefinic double bond is conjugated with an electron-withdrawing group, for example acrylic acid esters.
This addition is carried out in the presence of a basic catalyst chosen from magnesium oxide and basic anion-exchange resins. These resins are chosen from those having tertiary amines or quaternary ammoniums as functional groups.
If an anion-exchange resin is used, the reaction pressure is generally from 3103 to 6895 kPa.
Example II describes the addition of H.sub.2 S to methyl acrylate, without solvent, at a reaction pressure of 3102.75 kPa and at a temperature of 73.degree. C., in the presence of Amberlyst A-21 resin (Rohm Haas). This resin possesses dimethylamino functional groups. The selectivity indicated is 97.3% with a conversion of 100%, the H.sub.2 S/methyl acrylate molar ratio being 6.3/1.
Example IV shows that the selectivities obtained are higher than the selectivities obtained at lower pressure (1931 kPa) according to J56 147,763.
Moreover, patent EP 0,168,167 describes the preparation of a silica or of an alumina functionalized with tetramethylguanidine.
The silica or the alumina is reacted with 2-3-(triethoxysilyl)propyl!-1,1,3,3-tetramethylguanidine in order to graft this radical by formation of an --O--Si or --O--Al bond and removal of ethanol. The solid supports thus functionalized have been studied as basic catalysts in the transesterification.